Nuclear reactors have their fuel contained in sealed cladding for the isolation of the nuclear fuel from the moderator/coolant system. The term cladding, as used herein, refers to a zirconium based alloy tube. Often the cladding will be composed of various layers including a zirconium alloy substrate and an unalloyed zirconium barrier.
The cladding--nominally in the order of 0.030 inches thick--is formed in the shape of a tube with the nuclear fuel contained typically in pellet form therein. These pellets are stacked in contact with one another for almost the entire length of each cladding tube, which cladding tube is in the order of 160 inches in length. Typically, the cladding tube is provided with springs for maintaining the axial position of the fuel pellets and so-called "getters" for absorbing fission gases. Thereafter, the internal portions of the fuel rod are pressurized with helium to help conduct the heat from the fuel material to the cladding.
Zirconium and its alloys, under normal circumstances, are excellent for nuclear fuel cladding since they have low neutron absorption cross sections and, at temperatures below about 350.degree. C., are strong, ductile, extremely stable and relatively nonreactive in the presence of demineralized water or steam. "Zircaloys" are a family of corrosion-resistant zirconium alloy cladding materials. They are composed of 98-99% by weight zirconium, with the balance being tin, iron, chromium, and nickel. "Zircaloy-2" and "Zircaloy-4" are two widely-used zirconium-based alloys for cladding. Zircaloy-2 has on a weight basis about 1.2 to 1.7 percent tin; 0.12 percent iron; 0.09 percent chromium and 0.05 percent nickel. Zircaloy-4 has essentially no nickel and about 0.2% iron but is otherwise substantially similar to Zircaloy-2.
Splitting of Zircaloy cladding may occur due to the interactions between the nuclear fuel, the cladding, and the fission products produced during the nuclear reaction. It has been found that this undesirable performance is due to localized mechanical stresses on the fuel cladding resulting from differential expansion and friction between the fuel and the cladding. These localized stresses and strain in the presence of specific fission products, such as iodine and cadmium, are capable of producing cladding failures by phenomena known as stress corrosion cracking and liquid metal embrittlement.
To combat this problem, some cladding includes barrier layers having low neutron absorption formed on the tubing inner surfaces. Cladding containing barrier layers is sometimes referred to as "composite" cladding. The barrier layer is typically a moderately pure zirconium (such as sponge zirconium) or sometimes highly pure zirconium (such as crystal bar zirconium) sheath metallurgically bonded to the inner surface of the tubing. The pioneering work on barrier layer cladding is described in U.S. Pat. Nos. 4,200,492 and 4,372,817 to Armijo and Coffin, 4,610,842 to Vannesjo, and 4,894,203 to Adamson.
Barrier layers have been found to effectively prevent damage to the cladding due to interaction with the pellet. However, if the cladding wall is compromised in some manner (e.g. perforated or split), and water enters the fuel rod interior, the protection afforded by the barrier layer can be reduced. This is because the steam produced by water within the fuel rod can rapidly oxidize the barrier layer.
The mechanical initiation of a cladding breach can be attributed to various causes. A breach can start when debris such as wires or metallic shavings or particles find their way into reactor water that flows within the fuel bundles between the fuel rods. The debris may lodge at a fuel rod spacer adjacent the cladding wall. As a result, the debris vibrates or frets against the cladding wall under the influence of the passing steam/water mixture. Such vibration continues until the cladding wall is penetrated. Corrosion also can be the source of crack initiation and propagation. Moreover, manufacturing defects can be the points of crack origin. Still further, crack propagation can start on the inside of the fuel rods in the corrosive high pressure environment present during in service reactor life.
To protect the zirconium barrier from oxidation should a cladding breach occur, it has been proposed to use a three layer structure. In addition to the substrate and zirconium barrier, a corrosion resistant inner liner bonded to the fuel side of the barrier is employed. Typically, the inner layer will be made from a Zircaloy. If the cladding is breached and steam forms in the fuel rod interior, the inner liner will protect the barrier from rapid oxidation. Although this three layer design represents a significant advance, certain problems remain. For example, when exposed to fission products, Zircaloy inner liners sometimes serve as a site for crack initiation and propagation. If a crack in the inner liner becomes sufficiently deep (achieving a "critical length" or "critical depth"), it can propagate through the zirconium barrier and possibly through the entire cladding. It should be noted that the terms "critical length" and "critical depth" used herein refer to cracks in the radial direction of the liner wall.
Thus, there exists a need for cladding having an inner liner which protects the barrier layer from oxidation and at the same time resists crack initiation and propagation at the cladding fuel side.